TIMING OF ALLEGHANIAN TECTONICS DETERMINED BY CENTRAL APPALACHIAN FORELAND BASIN ANALYSIS DONALD R CHESNUT, JR. Kentucky Geological Survey 228 Mining andMineral Resources Bldg. University of Kentucky Lexington, Kentucky 40506-0107, USA ABSTRACT A revised stratigraphic and structural framework of the Central Appalachian Basin is used to interpret the timing of tectonic activity during the Alleghanian orogeny. The stratigraphic framework indicates that clastics with an eastern source were deposited in a basin subsiding to the east in the Late Devonian and in a very slightly subsiding basin in the Early Mississippian. During mid- Mississippian time, clastic sedimentation and basin subsidence were minimal, and a broad carbonate platform was developed. During the latter part of the Late Mississippian, clastic deposition and basin subsidence were renewed and persisted into the Permian. Subsidence was greatest to the southeast. The occurrence of the clastic wedges and timing of basin subsidence, as well as the occurrence of a regional unconformity, structural features, and tonsteins, are. related to events occumng in the Alleghany orogen. The timing of these basinal features is compared with tectonic events reported in the literature. Subduction along the eastern margin of North America during the latest Mississippian through Middle Pennsylvanian time is suggested. Subduction was followed by continent- continent collision during the Early Permian. INTRODUCTION The Alleghanian orogeny was the last great orogeny to affect eastern North America. The timing of the events that occurred in this orogeny is controversial. However, the response to Alleghanian tectonism in the craton and foreland basin can be used to infer the timing of events within the orogen itself. Large-scale tectonic events must affect crustal rocks in the vicinity of the orogen. Clastic wedges and basin subsidence among other responses are assumed to occur at about the same time as tectonism. In this paper evidence from a new stratigraphic and structural framework Of the Carboniferous rocks of the Central Appalachian Basin (Figure 1) is examined to infer the timing of Alleghanian orogenic events. Stratigraphic and Structural Frameworks A new lithostratigraphic and structural framework of the Central Appalachian Basin, based on detailed cross sections constructed through the Carboniferous rocks, was developed by Chesnut (1988, in press A). The cross sections were constructed from descriptions of approximately 2,000 boreholes and oil and gas wells collected from coal companies, land companies. state geological surveys, 203 ..a ..a published literature, and engineering reports. Thickness trends, regional unconformities, and onlap/offlap relationships of units in these sections (Chesnut, 1988, in press A) have been used to infer stratigraphic relationships that aid in determining the timing of Alleghanian orogenic events. Results from this study and from the published literature are summarized in the following section. Figure1. Location of the Central Appalachian Basin in the eastern United States. REGIONAL FEATURES OF THE CENTRAL APPALACHIAN BASIN Some features in the Appalachian foreland basin that record Alleghanian stresses or other Alleghanian events occurring in the orogen are difficult to date. These include (a) development of jointing, (b) remagnetization of red beds by movement of orogenic fluids (Miller and Kent, 1988), (c) twinning of calcite crystals in carbonate rocks (Craddock and van der Pluijm, 1989), (d) cementation patterns in carbonate sequences (Nelson and Read, 1990, fig. 4), and (e) development of pedogenic slickensides (Gray and Nickelsen, 1989). However, other basinal features such as clastic wedges, basin subsidence, the presence of regional unconformities, fault and arch movements, tonsteins and bentonites, and the dating of fluid migration can be used to infer the timing of events within the orogen. 204 Clastic Wedges and Basin Subsidence Clastic wedges in sedimentary basins record the influx of sediment from uplifted source areas. The variation in thickness of the wedges can be used to infer the accommodation space of sediment deposited, and from this the degree of differential basin subsidence can be inferred. The first clastic wedge occurring in the Carboniferous was deposited from the Late Devonian through Early Mississippian (Figure 2). The Chattanooga Shale (Late Devonian and Early Mississippian) and the Borden (Grainger and Maccrady Formations) and Fort Payne Formations (both Early Mississippian) represent a largely siliciclastic sequence in the Central Appalachian Basin (Figure 3a). Based on grain size and paleocurrent indicators, the dominant clastic source for the Chattanooga and Borden was from the east (Kepferle, 1977; Ettensohn, 1985). Thickness trends of the clastic wedge indicate that basin subsidence was greater to the east and had a strike oriented roughly north to south (Dillman, 1980; Chesnut, 1988; in press A). Basin subsidence was greater during the Late Devonian and much reduced during the Early Mississippian (Figure 3c), as indicated by the greater thickness variations from basin margin to axis of the Late Devonian sediment package compared to the Early Mississippian package. The Late Devonian and Early Mississippian clastic wedges are overlain by the middle and Late Mississippian Slade Fonnation and equivalent units (Figure 2). This thick, widespread carbonate unit marks a time of very reduced clastic input (Figure 3b) and subsidence in the Central Appalachian Basin (Figure 3c). A return to siliciclastic deposition is represented by the Pennington Group (Late Mississippian) and overlying Pocahontas (Early Pennsylvanian), Breathitt (Early and Middle Pennsylvanian), Conemaugh (Late Pennsylvanian), Monongahela (Late Pennsylvanian) Formations, and the Dunkard Group (Early Permian in part)(Figures 2-3a). Grain-size trends indicate a source to the east and southeast for most of the units and a source to the northeast for the Lee Formation (Figure 2)(Bement, 1976), which occurs within the lower part of the Breathitt Formation. Thickness trends of subunits within the Pennington Group and Breathitt Formation indicate greater basin subsidence (Figure 3c) to the southeast, with a strike oriented northeast-southwest (Chesnut. 1988, in press A). Post- Paleozoic erosion has reduced the occurrence of the Conemaugh, Monongahela, and Dunkard in the study area to the Allegheny Synclinorum (Parkersburg Syncline) in northeastern Kentucky, northwestem West Virginia, and southeastern Ohio. Thickness trends of these Late Pennsylvanian and younger units suggest that the Allegheny Synclinorum in and near Kentucky may have been an actively subsiding feature during the Late Pennsylvanian and Permian (Figure 3c)(Sergeant, 1979; Chesnut,1988, in press A). Early Pennsylvanian Unconformity Regional unconformities indicate periods of relative basin uplift or eustatic regression, when basins are exposed and streams erode to a lower base level. Two thick conformable stratigraphic sequences, separated by a discontinuity, are recognized in the Carboniferous rocks of the Central Appalachian Basin. The lower sequence includes the Pocahontas Formation (Early Pennsylvanian) and underlying Mississippian strata (Pennington Gmup), whereas the upper sequence consists of all Pennsylvanian strata above the Pocahontas Formation. The discontinuity between these sequences is the. Early Pennsylvanian unconformity (Figure 3d), previously called the “Mississippian-Pennsylvanian unconformity” in much of the literature (e.g., Rice and others, 1979). Rocks of the lower sequence are sequentially truncated to the northwest as low as the Borden Formation (Early Mississippian). The overlying Pennsylvanian sequence onlaps the unconformity to the northwest up to the top member of the Lee Formation (Early to lower Middle Pennsylvanian)(Chesnut, 1988, in press A). The contact of the lower and upper sequence is apparently conformable in the southeastem part of the basin in western Virginia and southern West Virginia (Englund and others, 1979). The increase in trucation below the unconfonnity and onlapping above the unconformity toward the northwest, coupled with the probable occurrence of a conformable relationship between the sequences near the axis of the basin, indicates relative uplift (or failure to subside) of the northwestern margin of the Central Appalachian Basin during the Early Pennsylvanian. This unconformity is described in more detail in Chesnut (1988, in preparation). Uplift of Cincinnati Arch Laskowski and others (1980) provided a 337±7 Ma Rb-Sr age (mid- 208 Mississippian) for authigenic glauconite in the Brassfield Formation (Early Silurian) of Ohio (Figure 3e). This age is considerably younger than the 410 Ma Early Silurian Brassfield Formation. Laskowski and others (1980) attributed the younger age to closing of fluid exchange caused by “...changes in fluid-pressure gradients and transport pathways produced by an uplift of the Cincinnati Arch.” Although Early Mississippian strata do not appreciably thin toward the Cincinnati Arch, Late Mississippian and Pennsylvanian strata on both sides of the Arch do thin (Chesnut, 1988, in press A; Furer, 1989). Several episodes of uplift focused along the arch probably occurred. Movement Along Other Structural Features Sediment thickness changes and offset of Carboniferous units can be used to infer movement along structural features in the Central Appalachian Basin. Movement along these structures might be related to cratonic